The present invention generally relates to ultrasonic welding. More specifically, the invention disclosed herein pertains to decoupling an ultrasonic transducer from the direction of working displacement of an ultrasonic welding assembly, using a transverse mounting arrangement, and driving the welding assembly to create system resonance at the nodal region to take advantage of the Poisson Effect.
Ultrasonic welding is a technique employed for joining thin, malleable materials, such as thermoplastics and even soft metals like aluminum and copper. In industry, ultrasonic welding is a good automated alternative to glues, screws or snap-fit technologies typically used to join materials. The benefits of ultrasonic welding are that it is much faster than conventional adhesives or solvents. The welding time is very quick, and the pieces do not need to remain in a jig for long periods of time waiting for the joint to dry or solidify. The ultrasonic welding process can easily be automated, making clean and precise joints that rarely require any touch-up work. The low thermal impact on the materials involved enables a greater number of materials to be welded together, as well. Moreover, because no glues or other additives are employed during the process, ultrasonic welding is a great choice for food-based package welding, such as plastic or aluminum bags like the kinds employed for chips and other snacks.
During the ultrasonic welding process, parts are laid together between a fixed shaped nest (called an “anvil”) and a sonotrode (called a “horn”). The sonotrode is connected to a transducer and booster assembly (commonly referred to as a transmission line or stack), which is used to convert electrical energy into acoustic vibrations. Such low-amplitude acoustic vibration is emitted from the sonotrode and into the materials being welded at the intended joint location. Typical frequencies used in ultrasonic welding range from 15 kHz to 40 kHz, but sometimes may even be found as high as 100 kHz. The ultrasonic energy melts the point of contact between the parts, creating the joint. Ultrasonic welding works by causing intense contact stress which induces melting of the material(s) due to absorption of vibration energy which are introduced across the joint to be welded. To ensure the welding joint is in the desired location, and is of the proper size, the interface of the two materials may be specially designed to concentrate the melting (welding) process. Although some heating in the joint area does occur, it is usually not enough to melt the materials, and instead it is the vibrations introduced along the joint being welded that causes the materials to weld together.
The applications of ultrasonic welding are extensive and are found in many industries, including electrical, computer, automotive, aerospace, medical, and packaging. Whether two items can be ultrasonically welded is determined by material composition, joint design, and tooling. In film-to-film applications, such as in welding the seals on snack bags and the like, if the materials are too thick the ultrasonic welding process will not join them. Advantageously, the ultrasonic welding process is very fast and easily automated, with weld times often below one second. Also, there is no ventilation system required to remove heat or exhaust, which also helps reduce overall manufacturing costs. In addition, ultrasonic welding is excellent for assemblies that are typically too small, too complex, or too delicate or dangerous for more common welding techniques.
The food industry finds ultrasonic welding preferable to traditional joining techniques because it is fast, sanitary, and can produce hermetic seals. An exemplary conventional ultrasonic welding assembly 100 is illustrated in FIG. 1. In this conventional assembly 100, an ultrasonic sonotrode 110 is included to provide the acoustic vibrations for ultrasonic welding. The acoustic vibrations are introduced to the sonotrode 110 using an ultrasonic transducer 120, and are propagated along the longitudinal axis L1 of the assembly 100. As mentioned above, the ultrasonic transducer 120 converts an electrical input 125 into acoustic waves, and the acoustic waves may then be amplified using a booster 130. The ultrasonic sonotrode 110 includes a welding surface 115, which in this example is a welding edge 115 that contacts a material 140 to be ultrasonically welded while the acoustic vibrations are propagating through the sonotrode 110 towards the welding edge 115.
To ultrasonically weld the material 140, the sonotrode 110 oscillates from the acoustic waves propagating therethrough while the welding edge 115 is moved to contact the material 140 and compress it against an anvil 150. As illustrated, in conventional assemblies 100 the transducer 120, booster 130, and direction for welding using the welding edge 115 of the sonotrode 110 are in-line with the longitudinal axis L1, and therefore the acoustic waves propagate along a single axis L1 throughout the entire assembly 100. The oscillation of the welding edge 115 onto the material 140 while pressing the material 140 against the anvil 150 causes the material 140 to be ultrasonically welded.
Unfortunately, because the transducer 120 and the welding edge 115 are positioned along a single longitudinal axis L1, premature failure of the ultrasonic welding assembly 100 may occur. More specifically, since the oscillating welding edge 115 is pressed against the anvil 150 (with the material 140 therebetween) during ultrasonic welding, vibrational feedback (i.e., impact pulses) caused from the physical contact of the oscillating sonotrode 110, material 140, and anvil 150 propagates back through the sonotrode 110, through the booster 130, and finally back into the transducer 120. The vibrations fed back into the transducer 120 consistently leads to premature transducer 120 failure. In addition, the linear arrangement of conventional ultrasonic assemblies occupies a large amount of space within an ultrasonic welding apparatus. Moreover, such a linear arrangement requires the assembly 100 to move towards and away from the anvil 150 for each incidence of ultrasonic welding of the material 140. Such movement is not only time consuming, but also requires additional machinery and energy for repeatedly moving the assembly 100 back and forth for each welding operation. Such additional equipment and energy also results in increased costs and the potential equipment failure in such conventional approaches. In view of such deficiencies, there exists a need in the art for an improved ultrasonic welding apparatus and method that does not suffer from the deficiencies found in conventional ultrasonic assemblies.